The present invention provides for an improvement in a cold gas-dynamic spraying process comprising purifying the spent carrier gas used in the spraying process. More particularly, the present invention provides for the use of a ceramic filter to purify and recycle the carrier gas in a cold gas-dynamic spraying process.
Various methods are employed to coat metallic surfaces. These methods vary as the type of coating and its purpose, be it decorative or for corrosion resistance. One process is cold gas-dynamic spray coating which involves spraying small powders at high velocities towards the substrate to be coated. On impact, the small particles which are typically metallic plastically deform on the substrate and form a metallic bond. This is a xe2x80x9cno-heatxe2x80x9d process and differs from competing processes such as thermal spray, plasma spray and electroplating which all require heat and/or dangerous chemicals which can limit applications.
In principle, the cold, gas-dynamic spray process relies on kinetic energy metallization (KEM) which utilizes powders having diameters of 1 to 50 microns. This is in contrast to other thermal processes where particles of 50 to 200 micron diameters are employed. These larger particles would bounce off the substrate rather than plastically deforming and forming the metallic bond.
Helium gas is typically employed as the medium to carry the particles because it has a high sonic velocity and can achieve particle speeds sufficient to achieve the metallic bonding. The process will typically operate between 150 to 300 psig and can run up to 500 psig. The helium flow rates are in the range of 150 to 200 Scfm with a powder flow rate of 5 to 15 kg/hr when the particle size is equal to 1 to 50 microns. Maximum level of solid does not exceed 20% of the helium flow rate. This results in a deposition efficiency of about 25 to 40% being the amount of powder fed into the system over the amount actually deposited.
This will result in an impure helium stream leaving the spraying chamber at temperatures between 100 and 400xc2x0 C. Typically this helium is fed to a scrubber and discharged to the atmosphere. However, the present inventors have discovered a process whereby the used helium can be cleaned and recycled back for use in the spraying chamber. This results in improved efficiency and cost savings as the amount of helium employed in the process decreases.
The present invention provides for an improved cold gas-dynamic spray-coating process wherein the improvement comprises directing the used, particle-laden carrier gas to a high temperature ceramic filter. The ceramic filter will separate the exhaust carrier gas from the particles and a gas analyzer will determine the purity of the carrier gas. If the carrier gas meets requisite purity it is then recycled back to the powder reservoir mixing device and into the spraying chamber with no further treatment.
If the carrier gas is not pure enough for spraying, a multi-valve assembly will direct the carrier gas through a heat exchanger and then to a gas purification unit, such as a pressure swing adsorption or membrane system to separate the carrier gas from oxygen, nitrogen, water and other inert gases.